Brandie Tarvin (9/26/2013)
Do not, under any circumstances, put them on the same DBs / same instance regardless of what services you are providing. It will be a maintenance nightmare, not to mention a legal one if one client gets ahold of another client's data.
With a proper database design and security protocols and policies in place a multi-tenant database is not a problem. In fact it is a more scalable solution for many scenarios. What if you have thousands of customers (imagine dealing with thousands of databases or instances! !#$@?*).
Granted with separate instances your security can be more lax. But with proper application design (separation of data access layer from presentation), encryption and leveraging best security practices it really a non-issue.
The probability of survival is inversely proportional to the angle of arrival.